You are christian when you keep going to church and take part in holy days or when you act as a christian.
This is an important point. There are many many people in my country who will write "christian" on a census form, but who only go to church for weddings and funerals.
communism was a bastion of atheism
I think you mean that the other way around 
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Because "communism was a bastion of atheism" means you have to be a communist in order to be an atheist, which is obviously not true. It has traditionally been the other way round: in (e.g.) USSR, PRC, religion was discouraged.
no
if communism is a bastion of atheism, it means most of atheists are communists, that you have to be an atheist in order to be a usual communist
if you say atheism is a bastion of communism it would mean all atheists are communists which isn't true, there is plenty of atheists that despise communism while all popular communists are atheists